Patent GB-543048-A (GB)
Title: Phase-modulated telegraph system
IPC: H04L27/227
Filed: 1940-03-08
Abstract: 543,048. Code telegraphy. STANDARD TELEPHONES & CABLES, Ltd., RALPH, F., and FAIRLEY, F. March 8, 1940, No. 4362. [Class 40 (iii)] Secret alternating-current signals, in which marks are distinguished from spaces by a change of phase so small that resulting transients can be rendered unnoticeable, are sent over the same circuit as amplitude-modulated signals of the same frequency or of more than one frequency and the apparatus is such that no switching operations or warning signals are required for the change from phase-modulated to amplitude-modulated signals ; thus the fact that a secret channel exists can be concealed from a spy who taps the signals. In the embodiment shown, ordinary signals of voice frequency f pass to line 8 from the oscillator 1 through an attenuator 9 and band-pass filter 10, and are modulated by means of a shortcircuiting key K2. Current from the oscillator 1 also passes through an amplifier 2 and frequencytrebler 3, so that current of frequency 3 f goes to line 8 through the band-pass filter 7 after being modulated either by the short-circuiting key K3, which produces ordinary amplitudemodulated signals, or by the key K1, which effects a small change of phase, the impedance elements 4, 5, 6 being so dimensioned that the change of phase is not accompanied by change of amplitude and is therefore unnoticed by any spy who taps into line 8, transients being suppressed by the filters if the phase-change is less than 30 degrees ; moreover the spy is misled by hearing interspersed non-secret messages from keys K2 and K3. At the receiver, the non-secret messages reach the telephone receiver 18 through a hybrid coil 13, those of the fundamental frequency f passing through a frequency-trebler 20 and band-pass filter 22. The secret or phase-modulated signals reach the hybrid coil through the filter 12, and are accompanied by continuous oscillations of the fundamental frequency f which pass through filter 11, variable-gain amplifier 19, frequency-trebler 20, phase adjuster 21, and filter 22 to the other side of the hybrid coil 13. The phase and amplitude of these concomitant oscillations are so adjusted that the phase-modulated oscillations are neutralized and nothing is heard in receiver 18 until the phase is shifted by depression of the key K1, when unbalance is effected and a signal is heard in the receiver. In modifications, (1) two-way communication is established by the use of a further pair of frequencies for the return signals ; (2) the range from 500 to 2.200 c./sec. is used for superimposed telephony, fundamentals below and harmonics above that range being used for telegraphy; (3) instead of the hybrid coil 13 and telephone receiver 18, a cathode-ray tube may be employed, the phasemodulated and concomitant carriers being applied to different pairs of plates so that the oscillographic trace changes its form when a change of phase occurs ; (4) for detecting the phasemodulation, concomitant unmodulated carrier may be dispensed with if a phase-shift of 2#/n. where n is a whole number, is employed. The nth harmonic of the signal frequency is generated from the received signals and is used to synchronize a local oscillator, which produces oscillations of the signal frequency for the purpose of phasecomparison ; (5) a phase-shift of #/n is used in the line, and at the receiver the nth harmonic (whose phase shift is #) is produced and is compared with a locally generated oscillation of the same frequency, which is synchronized by means of the 2nth harmonic generated from the signals ; (6) a phase-shift of #/3n is used, with the 3nth harmonic (having a phase-shift of 60 degrees) for signal current and the 6nth harmonic for synchronizing current, particularly in automatic transmission ; (7) in the cathode-ray method of reception, the invariable harmonic may be applied to one pair of plates and any other suitable harmonic to the other pair.